The determination of an animal’s sex is often thought of as a purely genetic process, dictated by chromosomes inherited from parents. However, nature presents a fascinating alternative where the environment plays a decisive role. In some species, the temperature experienced during a specific developmental window dictates whether an embryo develops into a male or a female, a phenomenon known as temperature-dependent sex determination.
Understanding Temperature-Dependent Sex Determination
Temperature-Dependent Sex Determination (TSD) is a form of environmental sex determination where the temperature during a specific period of embryonic development dictates the sex of the offspring. This contrasts with genetic sex determination, such as the XX/XY system found in mammals, where sex is fixed at fertilization by sex chromosomes. In TSD, the physical signal of temperature is transduced into a biological signal that ultimately determines gonadal sex.
Different temperature ranges during incubation lead to the development of different sexes; for instance, cooler temperatures might produce males, while warmer temperatures produce females. The precise mechanisms are still being investigated, but evidence suggests that temperature acts on genes coding for steroidogenic enzymes and sex steroid hormone receptors, particularly the enzyme aromatase, which converts male hormones (androgens) into female hormones (estrogens).
Animals Where TSD Occurs
TSD is observed across various animal groups, primarily reptiles and some fish. It is nearly universal in crocodilians, widespread among turtles, and present in some lizards, such as the Tuatara. This contrasts with many snakes and lizards where sex is chromosomally determined.
The patterns of TSD vary among species. One common pattern (Pattern IA) found in many turtles, like the red-eared slider, is that low temperatures produce males, and high temperatures produce females. Another pattern (Pattern II or FMF) is seen in some turtles, lizards, and alligators, where females are produced at both low and high temperatures, while intermediate temperatures yield males.
The Role of Temperature and Environmental Factors
Temperature’s influence on sex determination is precise, operating within a “critical period” of embryonic development. This period, also known as the thermosensitive period (TSP), is when the gonads are undifferentiated and highly sensitive to temperature changes, typically spanning the middle third of incubation.
A key concept in TSD is the “pivotal temperature,” which is the constant temperature that produces an equal 1:1 ratio of male to female offspring. For instance, in red-eared slider turtles, temperatures below 28°C produce all males, and above 31°C, all females, with the pivotal temperature falling within the narrow range in between. Small changes, often just 1-2°C, within this transitional range can lead to shifts in sex ratios. While temperature is the primary factor, other environmental elements like nest depth and moisture can indirectly influence the incubation temperature and, consequently, the sex ratio of the clutch.
Conservation Concerns
The reliance on temperature for sex determination presents conservation challenges, especially in the face of global climate change. Rising global temperatures can skew sex ratios, leading to a disproportionate number of one sex being born. For species where warmer temperatures produce females, such as many sea turtle species, prolonged heat waves can result in overwhelmingly female populations.
A study on the Great Barrier Reef’s green sea turtles found that 99% of immature turtles were female, indicating a significant increase in female hatchlings in recent years due to warming trends. If this trend continues, a lack of males could threaten population viability by reducing reproductive opportunities and limiting genetic diversity. Conservation efforts for these species must consider the impact of temperature on sex ratios, as rapid climate change may outpace the species’ natural ability to adapt.